I read this phrase in a magazine : 'Je pique des crises de nerfs' which I take to mean 'I am afflicted by nervous crises'. I am wondering why it is 'des crises' and not 'de crises' (as it is 'de nerfs' and not 'des nerfs')?
Crise de nerf is a colloquial expression that is used here in the idiom piquer une crise de nerf.
It is a colloquial way to say "to become suddenly upset, to freak out over something or someone".
Similar idioms are:
Piquer une tête: to dive
Piquer un fard: to blush
Piquer un fou rire: to have an incontrollable laugh
Des is used simply because it is the plural of une, there is more than one crisis. English would uses no article here. (singular: I have a nervous crisis / plural: I have nervous crisis)
De is not used here because it is not the plural of un(e).
Let's take care of the basic question first; why "crise de nerfs" instead of "crise des nerfs".
(TLFi) − Spéc. Crise de nerfs. ,,Expression populaire pour désigner des manifestations paroxystiques à caractères neuro-psychiatriques et comportant plus particulièrement des manifestations psychomotrices, et des troubles de la conscience et du comportement`` (Lafon 1969).
In this locution there is no article ("des" is the contraction of "de" (preposition) and "les" (article)) because the semantic relation between "crise" and "nerfs" is one that has been reckoned with as that of kind (sort, type,…) rather than another relation.
- "engorgement des amygdales", "inflammation des yeux, des intestins, des veines, des parties, des tissus, etc."
because the various parts are being affected; it's is different with "crise de nerfs" as the nerves are considered to be the agents of the affliction rather than being afflicted. But there is in this remark no solid principle, and as shows the Google page relative to the use of "des" (mentioned above), people do not go by any such principle.
So, a why has no easy answer, and it is more useful to consider that this is merely usage, and at that, it is changing.
The second question has a complicated answer. As is the sentence, "des" is necessary because "de" in this positive contexte comes through as the preposition "de", and the verb "piquer" is "transitif direct" (has an object introduced by no preposition). Since, the sentence aims at saying that the person is afflicted by a certain number of nervous fits, what is called for is an indefinite article (zero article in English, or "some"). There is just one choice in french: "des".
However, if we change the sentence only slightly by just making it negative, then "de" is the right word.
- Je ne pique pas de crises de nerfs.
In this case, we are not dealing with the preposition "de" but with the article "de". This article is necessary for the negative form. This reference, Guide de grammaire française pour étudiants finnophones, l'article indéfini, explains that well (never mind the Finish, or the few details concerning it that are in French).
So, in the negative form this is the proper article, but there are exceptional cases of use of the negative form when you have to revert to the usual "des". Those are so called cases of "partial negation" (also explained in the reference above).
- je n'ai pas des crises de nerfs, j'ai des crises de foie. (the verb "piquer", colloquial, not free of connotations, is unidiomatic for "crise de foie".)